Fix to change.log

With the migration to .rst from .md, the Issues Resolved and
Known Issues sections for v1.4 were using Header 1 format.

This patch changes to using Header 2 for these sections.

Change-Id: Ic3127d84eb169a65039fd4cc8284c6429302732d
Signed-off-by: David Cunado <david.cunado@arm.com>

docs/change-log.rst

@@ -266,7 +266,7 @@ New features
    pre-empted SMC during PSCI power management requests.
 
 Issues resolved since last release
-==================================
+----------------------------------
 
 -  ARM TF can be built with the latest mbed TLS version (v2.4.2). The earlier
    version 2.3.0 cannot be used due to build warnings that the ARM TF build
@@ -280,7 +280,7 @@ Issues resolved since last release
    shutdown request using the PSCI SYSTEM_OFF API.
 
 Known Issues
-============
+------------
 
 -  Building TF with compiler optimisations disabled (-O0) fails.